It seems like such a simple question, but it's actually one of the biggest mysteries in all of science. Is it because of how our brains are organized, how ancient humans gripped tools, or is it simple anti-lefty prejudice?

Nobody knows for sure, but scientists have come up with theories that are alternately intriguing, persuasive, and a little bonkers. Let's take a look at the often baffling science of handedness, running from how the development of language in early humans might have helped cause the evolution of handedness...to how all the world's languages seem to have it in for lefties.

Why does handedness exist?

Handedness - the idea that one hand is better able to perform certain tasks than the other - is, if not exclusively a human trait, then certainly a mostly human one. After all, how could you tell if a dog was left-handed or a lion was right-handed? Their paws aren't evolved to handle complex tasks like our hands are, and there's no evidence that non-primates favor one limb over any other.

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But exactly why humans favor different hands, or why most people tend to be right-handed, remains mysterious. The most common answer is that handedness is determined by the structure of our brains, which are divided into two hemispheres. Our brains are far more specialized than those of other animals, with different regions of the brain responsible for different specific tasks. Admittedly, these are only general guidelines, and most neural activities are shared between the hemispheres to some extent, but we can definitely say that many functions are primarily handled by one hemisphere as opposed to another. This is known as brain lateralization.

Two of the most energy-intensive human activities are language and the use of our fine motor skills - in other words, the use of our hands. One theory suggests that it's more efficient for the brain to cluster control of these two major tasks in one hemisphere rather than having it spread throughout the brain. Since the vast majority of people have their language functions centered in the left hemisphere, it follows that most people's fine motor skills would be controlled by the left hemisphere too. Each hemisphere generally controls the opposite side of the body, so the end result is that most people are right-handed.

However, the opposite does not hold true - being left-handed does not mean the language centers are located in the right hemisphere, which is fairly rare. Certainly, lefties are more likely than righties to have their right hemisphere responsible for language, but it's still not a common arrangement. Between 61 and 73% of lefties have their language centers in the left hemisphere, compared to over 90% of right-handed people.

This doesn't necessarily invalidate the division of labor theory. After all, between 70 and 90% of people are right-handed, and well over 90% of those people do indeed cluster language and fine motor skill control in the left hemisphere. What we're looking at here is the evolutionary equivalent of a rule of thumb. People's brains are generally organized to maximize energy efficiency, but a reasonably large minority of people - including most lefties - get along just fine with a less efficient arrangement.

But why are most people right-handed?

As we've discussed, another possible way of phrasing that is, "Why is the language center usually in the left hemisphere of the brain?" After all, if the energy intensive language centers happened to evolve so that they were usually in the right hemisphere, then most people would probably be left-handed instead. To that point, there's no reason why our brains couldn't have evolved that way - it's simply a historical fact that they didn't.

But that hasn't stopped scientists from attaching great significance to the fact that we evolved as righties instead of lefties. In the January 1, 2002 issue of Discover Magazine, Jocelyn Selim describes a particularly spectacular theory:

In most primates and other animals, the hemispheres of the brain divide the processing of tasks somewhat equally. But in humans, the hemispheres tend to specialize: Nearly all righties process language in the left side of the brain, while many lefties process language on the right. Because handedness and language both seemed uniquely human traits, biologists long assumed that they were closely linked.

One Oxford neurobiologist went so far as to argue that right-handedness could be traced back 200,000 years to a single mutation-a sort of genetic Big Bang that created hemispheric specialization, language, and higher cognitive functioning in one go. Right-handedness, to this way of thinking, is the most obvious mark of the genetic instructions that separate us from speechless, symmetric beasts.

While some of that that might be going a bit far - if nothing else, it doesn't seem to leave a lot of room for lefties - the root of handedness quite possibly is a random genetic mutation that pushed the language centers to the left hemisphere as the ancient human brain became more specialized. Without this particular genetic mutation, our brains still might have evolved to their present levels of sophistication, but our language hemispheres would be chosen at random, meaning handedness would be more evenly split.

In fact, a second major gene mutation might have had precisely that effect, at least in a subset of the population. University College London neuropsychologist Chris McManus suggests that sometime between 20,000 and 100,000 years ago, a second mutation entered the human gene pool that canceled out the brain's natural bias towards right-handedness, allowing for the emergence of more left-handers. People who carry this second mutation are also more likely to have unusual patterns of brain organization, which neatly explains why lefties are more to have both high intelligence and mental disorders.

So is it really all about language?

Well...that's just one theory. It's a pretty good one, as theories go, but it doesn't explain everything, and - as we've just seen - it very easily gets wrapped up in grand triumphal stories of humanity's brilliant evolution. Certainly, we should be careful in saying the development of language centers in the left hemisphere caused the rise of handedness. The two appear linked, but it may not be quite as direct as we might have imagined.

One issue is that we're not the only other animals to use a particular hemisphere of the brain to create noises. Birds and frogs, for instance, process the noises they make in one particular side of the brain, and there's no evidence of handedness in the way they use their limbs. University of Auckland researcher Michael Corballis suggests an alternative explanation for our handedness in the Discover article:

"It's quite possible that what set humans apart was that speech began from gestures, which would explain an indirect association with handedness. But it's one of those mysteries that refuses to resolve itself. Think of it this way: Primates do have very symmetrical brains, but then again, so did Einstein."

The division of labor theory and the two genetic mutations don't need to be thrown out - instead, we can roll them into a larger picture of the evolution of handedness. It's possible that the specialization of our brains were already pushing our species toward handedness, but this process was accelerated by the activities of ancient humans, in particular their use of tools. In a 2009 blog entry for Science, Michael Balter details one possible explanation for the rise of handedness:

The "technology-dense lifestyles" of early hominins might have required our ancestors to more or less make up their minds about what hands they were going to use to perform complex tasks. Moreover, such hand bias could have aided the learning process as hominins taught each other toolmaking and other skills; a number of studies have shown that people learn manually difficult tasks, such as knot-tying, more easily when they use the same left- and right-hand movements as their teachers.

We can also add into the mix the development of longstanding, pervasive social prejudices against left-handedness, which I'll discuss in more detail in a little bit. These prejudices likely influenced countless children who otherwise would have developed into lefties to essentially force themselves to be right-handed, effectively inflating the proportion of the right-handed population.

When did handedness first evolve?

We can feel reasonably confident that Neanderthals and Homo heidelbergensis were right-handed. Neanderthal skeletons tend to have stronger right arms and shoulders than their left counterparts, perhaps because they used them to throw spears while hunting animals. There's less direct evidence for Heidelbergensis, but we can tell from their preserved teeth indicate that they ate food with their right hands. The 1.6 million year old Homo ergaster skeleton Nariokotome Boy showed signs of right-handedness, but we can't infer the hand preference of an entire species from just one skeleton.

Determining the handedness of other primates is also tricky for a bunch of reasons. In general, wild primates aren't particularly likely to engage in the sorts of activities that require fine motor skills - and thus would require handedness - and it's hard to know whether we can really trust results we get from giving human-like tasks to captive primates. But as primatologist Bill Hopkins explains in the Discover article, decades of accumulated research have given us some rather unexpected answers:

A close look at primate research since the 1920s shows that all primates have hand preferences, and those preferences follow a clear pattern: Lemurs and other prosimians tend to be left-handed; macaques and other old-world monkeys are evenly split between lefties and righties; among gorillas and chimpanzees, 35 percent are lefties, while in humans that percentage hovers around 10. In other words, the more primitive the primate, the more likely it is to be a lefty. Left-handedness, far from being a recent invention, seems to predate right-handedness.

We know that the ancestor of all primates evolved about 65 to 85 million years ago as a member of the extinct order Plesiadapiformes. One of the last of the Plesiadapiformes was Carpolestes simpsoni, which had grasping digits like those of all its primate descendants. It's possible that the development of handedness goes right back to this very ancient transition from claws to digits.

Why primates apparently slowly moved from left-dominant to right-dominant as they evolved remains an open question. We could go back to the division of labor theory here. It's possible that because Carpolestes simpsoni and the earliest primates had small brains, they had to cluster control of their emerging motor skills in one hemisphere, which just happened to be the right.

This led to a lot of left-handed monkeys, but as larger, bigger-brained primates began to evolve, this division of labor became less important, which opened the door for a wider mix of righties and lefties. Of course, that doesn't explain why handedness started to swing towards righties as we get to chimps and gorillas is still - it's certainly possible that changing brain structure and random mutations played a major role, but we just don't know for sure.

What does it actually mean to be right-handed or left-handed?

After everything we've already discussed, that may seem likely a stupidly easy question, but it's surprisingly controversial. The everyday answer to that question is probably this: if you write with your right hand, then you're right-handed. It's a straightforward enough popular definition, but translating that into scientific terminology is trickier than you might think. Even simply saying that handedness is determined by which hand you prefer to use doesn't actually help us that much.

Let's consider some of the problems here. Should a person's dominant hand be determined by the hand they prefer to use, or the hand that performs better in tests? In other words, is handedness primarily psychological or physiological? Even if you can sort that out, there's still the question of how to categorize all this. Should left-handed and right-handed be considered precisely equal, or does the fact that such a vast majority of people are right-handed suggest that the people are simply either right or non-right? That's a bit of a charged way to look at things, but it does have some popularity in scientific literature.

And how about people who use different hands for different tasks? I write and throw with my right hand but bat and play(ed) hockey with my left hand - should I be considered primarily right-handed, a mix of right- and left-handed, or ambidextrous, meaning I feel equally comfortable using both hands? (Yeah...I'm probably not ambidextrous.) And should we look at handedness as something that can be lumped into only a very few categories - say true right-handed, more right-handed, ambidextrous/mix, more left-handed, and true left-handed - or something that exists on a broad spectrum?

Indeed, the very idea that one hand is "dominant" might be a complete misunderstanding of how our hands divide up tasks. In the Science post, Michael Balter details an argument by University of Liverpool archaeologist Natalie Uomini:

Uomini points out that handedness does not mean that one hand is "dominant" over the other. Rather, she writes, "both hands have different but equally important roles." In right-handed people, for example, the right hand might be used for tasks requiring greater manual dexterity whereas the left hand might perform the more mundane but nevertheless crucial role of supporting an object. (Imagine eating dinner with just a knife but no fork, for example.)

There isn't a ton of consensus here - ask a dozen scientists and you'll probably get at least ten different ways to define and organize handedness. Part of the problem here is that handedness is generally a pretty touchy issue, one with a long history of bias and prejudice. Speaking of which...

What exactly do people have against lefties?

For reasons that can probably best be described as "stupid", people have historically held left-handers in contempt. You can find evidence of an anti-left bias throughout the world's languages. The word "left" itself comes from the Anglo-Saxon word "lyft", which means "weak." We get the word "sinister" from the Latin word for "left", and that double meaning persists in the modern Romance languages.

It really is an insanely long list - English, French, Chinese, Korean, Finnish, Irish, Hungarian, Dutch, Spanish, Italian, Portuguese, German, and most of the Slavic languages all either link the word "right" with goodness, the word "left" with wrongness and impropriety, or both. That's not even getting into all the expressions and customs that have sprung up against left-handed people. In Ghana, even gesturing with the left hand can be considered taboo, and a common form of 19th century bigotry was to say minority groups such as homosexuals and Roman Catholics were left-handed.

As to why these prejudices exist - well, it does appear that majority right-handedness has been around at least since modern humans first emerged 200,000 years ago. This may simply be a case of a natural majority picking on a minority, perhaps out of an embarrassingly human fear of what's different. There's also at least one possible vaguely practical reason for this animosity. As any lefty will be happy to tell you, tons of everyday objects that right-handed people take for granted are a pain in the ass to use if you're a lefty. There's spiral notebooks, can-openers, stick shifts...and that's just what I remember from the Simpsons episode on the subject.

It's possible that ancient tools were similarly designed with right-handed users in mind, and the natural difficulties a lefty would have with such implements could draw social scorn and, over time, build up negative associations towards lefties. That all sounds fairly fanciful, honestly - although a few languages do have words that mean both "left" and "clumsy", suggesting a linked meaning somewhere along the line. Ultimately, we're probably just looking at good old-fashioned fear of the unusual.

There's really no definitive evidence either way that righties are better than lefties or vice versa - they just happen to hold their pencils differently. And while there have been a bunch of attempts to tease out specific differences between the two groups - including a recent study that basically suggested left-handedness is a form of cognitive impairment - there's way too much conflicting data out there to say much for certain.

That's the finding of a new study examining the link between cognitive ability and handedness. …
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As Chris McManus observed in the Discover article, ""The real question is why everyone wants left-handers to be defective", and there's a great deal of truth to that observation both throughout history and right up to now. Anyway, if this whole exercise has taught us anything, it's that we're probably just a couple random genetic mutations away from left-handers snootily trying to figure out what's up with that 30% minority of righties.